156 THE BREEDING OF ANIMALS 



unit characters increases, the number of possible different 

 individuals produced by crossing similarly increases. 

 In the case of tri-hybrids in which three characters com- 

 bine, the number of different individuals or zygotes pro- 

 duced would be represented by (3 plus I) 4 . For the 

 purposes of the practical breeder, therefore, little progress 

 is possible except in cases where one or two characters 

 only are involved. In those cases in which the characters 

 involved are made of a combination of many units it is 

 first necessary that the important unit characters be 

 separated by long-continued breeding; in other words, 

 until it becomes homozygous. By such methods, it is 

 possible to obtain recombinations, and from the point 

 of view of the practical breeder, entirely new characters. 

 The mendelian principle may, therefore, in this way be- 

 come a powerful instrument in the future improvement of 

 domestic plants and animals. 



